2. Sift the flour twice with 1/2 cup of the sugar.
3. With an electric mixer on high speed, beat the egg whites, cream of tartar and salt until soft peaks form when the mixer is removed from the batter.
4. Add half of the remaining sugar and beat for 1 minute. Add the remaining sugar, 2 tablespoons at a time, beating after each addition.
5. Stir in the vanilla, and add lemon zest.
6. Fold the flour and sugar mixture into the egg whites, 1/4 cup at a time, until just incorporated.
7. Put the batter in an ungreased 10" tube pan and bake until the cake is a light golden brown and springy to the touch, about 1 hour. Invert the pan and let the cake cool completely before removing from the pan.
8. While the cake is baking, puree the berries in a blender with the lemon juice. Blend in the sugar by the tablespoonful, tasting after each addition, until the desired degree of sweetness is reached.
9. Strain through a fine strainer, pressing with a rubber spatula to release the juices.
10. Place 1 slice of cake on each of 6 plates. Spoon the blueberry sauce on and around the cake.